1. Field of the Invention
The present invention relates to a semiconductor device having improved interfacial quality of a semiconductor layer made of active polycrystalline silicon, and a method of manufacturing the same.
2. Description of the Related Art
An example of conventional semiconductor devices comprising a semiconductor layer made of active polycrystalline silicon is a thin film transistor having the structure shown in FIG. 5.
This thin film transistor comprises a channel forming portion 102 formed on an insulating glass substrate 101 to be held between a source region 103 and a drain region 104 formed on both sides thereof to form a semiconductor layer 112 made of active polycrystalline silicon. A gate insulating layer 106 made of silicon oxide is provided over the entire surface of the substrate 101 including the semiconductor layer 112, and a gate electrode 107 is provided opposite to the channel forming portion 102 through the gate insulating layer 106. A protecting film 108 is provided to cover the gate electrode 107 and the gate insulating layer 106, and a source electrode 110 and a drain electrode 111 are provided on the protecting film 108 to be connected to the source region 103 and the drain region 104, respectively, through contact holes 109 formed in the protecting film 108 and the gate insulating film 106.
The thin film transistor having the above-described structure has the probability that defects in the semiconductor layer are actualized in the interface between the semiconductor layer 112 and the gate insulating layer 106 to decrease carrier mobility of the semiconductor layer with increases in the gate voltage.
The present invention has been achieved for solving the above problem, and it is an object of the present invention to provide a semiconductor device capable of preventing deterioration in carrier mobility of a semiconductor layer, which is a quality of the interface between the semiconductor layer and an insulating layer, and a method of manufacturing the same.
A semiconductor device of the present invention comprises a semiconductor layer made of active polycrystalline silicon, an insulating layer made of silicon oxide, and an interface layer made of silicon nitride and provided between the semiconductor layer an the insulating layer.
The interface layer possibly compensates for lattice strain of the active polycrystalline silicon film due to the diffusion of nitrogen element of silicon nitride into the active polycrystalline silicon film, thereby satisfying desired quality of the interface between the semiconductor layer and the insulating layer. It is also possible to simultaneously compensate for interface defects of the active polycrystalline silicon layer serving as the semiconductor layer and form the interface layer.
Furthermore, in a semiconductor device, particularly, in a thin film transistor, silicon oxide used in the gate insulating layer serving as an insulating layer has excellent insulation voltage property, and thus the insulating voltage property required for the thin film transistor can also be satisfied.
The interface layer must have a thickness of 5 nm or more in order to compensate for lattice strain of the active polycrystalline silicon film due to diffusion into the active polycrystalline silicon film. With a thickness of 10 nm, the interface layer has the sufficient effect of compensating lattice strain. With a thickness of over 10 nm, the plasma processing time required for forming the interface layer is increased, and the effect of compensating lattice strain is not improved.
Therefore, the thickness of the interface layer is preferably 5 to 10 nm.
The insulating layer may have a desired thickness.
A method of manufacturing the semiconductor device of the present invention comprises processing the surface of a semiconductor layer made of active polycrystalline silicon with plasma using ammonia gas and silane gas to form an interface layer made of silicon nitride on the surface of the semiconductor layer, and processing the interface layer with plasma by using nitrous oxide and silane gas to form an insulating layer made of silicon oxide.
This method is capable of securely nitriding the surface of the active polycrystalline silicon layer by plasma processing using ammonia gas to form the interface layer having desired interfacial quality. The method can also simultaneously compensate for defects of the active polycrystalline silicon layer and form the interface layer on the surface of the active polycrystalline silicon layer.
The plasma processing using ammonia gas and silane gas is preferably performed by discharge at a frequency higher than 13.56 MHz.
This is because the gases are decomposed, and at the same time, discharge energy causing no damage to the semiconductor layer made of active polycrystalline silicon to be nitrided is obtained.
In the method of manufacturing the semiconductor device of the present invention, the plasma processing is preferably carried out by using ammonia gas and silane gas with a bias voltage applied to an insulating substrate on which the semiconductor layer is formed.
This is because diffusion of nitrogen element of the interface layer made of silicon nitride into the semiconductor layer made of active polycrystalline silicon is accelerated by applying potential energy to the semiconductor layer and the interface layer.